The Pollinators' View - Insect Vision

Fewer than 20% of plant species on earth are pollinated by wind transport (notably grasses, ferns and conifers, but
also some angiosperms), about 2% by water transport.
All the rest are pollinated by
animals. Insects
(bees,
flies,
wasps, moths, butterflies,
beetles) are by
far the most common pollinators, but some
birds and
bats, and even a
few non-flying animals, also play a part.

Wind pollination works well when plant biodiversity is low, but animal (biotic) pollination is best where plants of the
same species are widely separated from each other. Scent and colour patterns unique to each species are used to
encourage a pollinator to specialize on a species, thus maximizing the success of cross-pollination.

In Canada, most pollination of wild plants is done by bees and wasps (Hymenoptera), with flies (Diptera) close
behind. Non-native honey bees (Apis mellifera) are relied on here to pollinate monoculture crops where native insects
have been extirpated by pesticides, but world wide,
80% of crop pollination
is carried out by native wild flies, bees and wasps.

Insects don't see light the way we do, so the visual cues they use, mostly flowers, appear different to them
than to us. In particular, almost no insect except day-flying butterflies and a few beetles sees red light, but
all see near ultraviolet which we can't. Most insects have only two photoreceptor types, so have little
ability to discriminate colours. However those that pollinate flowering plants, such as the honey bee and fruit
fly shown here, tend to have three as we do. Those few insects that can see red, flower searchers all, see four
colours. The butterfly shown has six receptors; two combine to form a broad blue peak, two others to form a red one.

bee-wasp eye sensitivity[0]

fly eye sensitivity

day-flying butterfly eye sensitivity

So, how might we see a garden the way pollinating insects do? With ultraviolet photography.

To start, I'm using a digital camera (Nikon D90) with sensor
filter removed, a UV-pass filter
(Baader Venus II)[3]
and a UV/IR-cut filter (B+W 486)[4]. Such a system doesn't see very much ultraviolet, as you can see below, but it sees enough to get started on my insect vision
quest[1]. Ultraviolet is mapped to red in the 'insect view' image so that blue
and green will stay as they are to us, to maximize familiarity. Unlike many published UV photos of flowers, the
brightness of each spectral segment is set equal in both human and insect views using a neutral grey card, rather
than exaggerating the UV.

Here is the spectral sensitivity of each channel of my D90, and the result of the filters:

dSLR sensitivity

insect camera 1.0 sensitivity

The red filter of the D90 sensor has twice the sensitivity to UV as the blue sensor, so it's the one to use. The response of the camera red sensor elements, although counter-intuitive, matches the sensitivity of
our eye pigments.
As you can see above, the equivalent, green, receptor of insects has a similar response. Obviously, it has an
evolutionary advantage, somehow.

The BBC can afford optical systems that take both UV and visible images at the same time. I can't, so begin by
stabilizing the flower with a stake, or picking it to put it within reach, and composing on a solid tripod. I then
take one photo with a filter that passes 400-700 nm, replace that filter with one that passes only
300-400 nm, adjust exposure by 7 stops, and take the UV image. The images are aligned, balanced and
combined in Photoshop.

The major difference between our eyes and those of insects is that while insect eyes have lower resolution than
human eyes, they are much more sensitive to movements within their visual field. Our eyes are designed to separate
objects in static views but insect eyes are designed to spot movement. Insects would not respond at all to still
photos. We use continuous small rapid movements of our eyes
(saccades) to
improve recognition of edges. Insects can't, but they do occasionally make
rapid flight adjustments when they need to
create movement for their eyes to resolve static objects in three-dimensional surroundings.

Notes:
[0] with all spectral plots here, the scale of each colour is chosen so that their integrals (total intensity) are
equal on each plot.
[1] I plan later to resurrect my 30-year-old film camera, a
Nikon FE, and look for films that see farther into the ultraviolet.
[3] The Baader Venus II filter is the only one I've found that has sufficient IR blockage to prevent non-UV
contamination of images given the high sensitivity of CMOS sensors up to 1100 nm. It's expensive though:
$260, 48 mm thread
only. And, since astronomers mount their filters the opposite way to cameras, the filter has to be reversed in its mount
so that the green side faces outwards.
Camerafilters.com has adaptors to standard camera sizes.
[4] The B+W (Schneider) 486 and Baader UVIR-cut filters are the only ones I've found that have sufficient IR & UV
blockage to prevent non-visible contamination of CMOS sensor images. The B+W 486 comes in most camera sizes; it's the one I use. The Baader comes
in 48 mm thread only; reversal is required as noted in [3].